Resistant organisms and antibiotic resistanceSpec 4.3
In short
Antibiotic resistance is an example of natural selection seen within a human lifetime. A mutation gives one bacterium an allele for resistance. The antibiotic kills the non-resistant bacteria, but the resistant one survives, reproduces and passes on the allele. Over time most of the population is resistant, so the antibiotic no longer works against the infection.
The emergence of resistant organisms is an example of natural selection that can be seen happening within a human lifetime. Bacteria reproduce very quickly, so changes in a population happen fast.
- In a population of bacteria, a mutation produces an allele that makes one bacterium resistant to an antibiotic. This is variation.
- The antibiotic is used. It kills the non-resistant bacteria, but the resistant bacterium survives. The antibiotic is the selection pressure.
- The resistant bacterium reproduces with less competition, and passes the resistance allele on to its offspring.
- Over time the population is mostly resistant bacteria, so the antibiotic no longer works against the infection.
This supports Darwin's theory because all the steps of natural selection can be observed: variation, a selection pressure, survival and reproduction of the better adapted, and inheritance of the useful allele.
Percentage of resistant bacteria (practice data)
In a lab, 300 bacteria were sampled before treatment and 12 were resistant to an antibiotic. After several rounds of treatment, 300 bacteria were sampled again and 192 were resistant. Calculate the percentage resistant at each time, and the change.
- Before: 12 ÷ 300 × 100 = 4%
- After: 192 ÷ 300 × 100 = 64%
- Change: 64 − 4 = 60 percentage points
Answer: 4% before, 64% after, an increase of 60 percentage points.
You may be asked to plot such data as a bar chart or line graph. Choose scales that use at least half the grid, label both axes with units, and plot the independent variable (time or treatment) on the x-axis.
Antibiotics do not cause the mutation, and bacteria do not 'get used to' antibiotics. The resistant bacteria were there by chance and are selected by the antibiotic.
Quick check
What causes new alleles to appear in a population?
Show answer
Mutation.
Give the four main steps of natural selection in order.
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Variation, competition so some survive and reproduce, alleles passed on, proportion with the useful allele increases.
In antibiotic resistance, what is the selection pressure?
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The antibiotic.
Why do populations of bacteria change quickly?
Show answer
Bacteria reproduce very rapidly, so the resistance allele spreads in a short time.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026